As NATO refocuses on its eastern borders after Russia's annexation of Crimea, the United States is quietly deploying more troops to train special forces in former Soviet bloc states anxious about Moscow's intentions.

Major exercises began last month in Poland, Slovakia and the Baltic states of Estonia, Lithuania and Latvia involving several hundred personnel from US special forces, the US European Command (EUCOM) said in a statement to Reuters.

Long-term plans include further training drills that will consistently keep about 100 US elite troops on the ground at any one time in NATO states close to Russia, with teams working in several countries, US official said.

The events in eastern Ukraine, where Russian-speaking insurgents using sophisticated weapons threaten to split the country, have put the whole former Soviet bloc region on alert and eager for NATO reassurance.

EUCOM says its Special Operations Command Europe (SOCEUR) increased the size and scope of its planned exercises after Ukraine flared into violence, reinforcing Washington's message to Moscow that it would stand by its allies.

"Training with our partners in their home countries is something that we have always done," said SOCEUR spokesman Lieutenant-Colonel Nick Sternberg. "The difference is that now we will maintain a (permanent) Special Operations Forces presence in theater along the eastern front of NATO on this training mission," Sternberg said.